Technical Article
The Protection of Silver
Against Atmospheric Attack
by P-A. Gay,* P. Berçot & J. Pagetti
Tarnishing of silver and its alloys in certain environments is a persistent problem. This paper explains
the various measures used to date to retard silver
tarnishing in various applications, such as jewelry,
the silversmith trade and electronics. Different antitarnish methods have been proposed by the scientific community, none of which perfectly satisfy the
requirements of all applications. A method of incorporating fine organic and/or inorganic particles into
silver-based electrolytic coatings is a very promising
solution. However promising, we find it still needs
further development.
The tarnish film appearing on silver surfaces limits the
application of silver-containing alloys in many fields.
Tarnishing is very harmful in electronic applications
because of the added electrical resistance. In decorative
applications, it ruins the workpiece appearance (color and
brightness). In addition, the weldability of silver deteriorates.
Nowadays existing processes will retard the natural
silver conversion but cannot completely eliminate it.1-7
Any solution to the tarnish problem has to be a compromise between three fundamental aspects: (1) surface
appearance, (2) contact resistance and (3) weldability.
There are several methods to protect silver against tarnishing. The most frequently used technique consists of
depositing a protective film (organic, mineral or metal) on
the silver object. Some protective methods are still under
development while the others have already found widespread industrial application. They can be grouped into
various categories.
Organic Deposits
Lacquers
The use of lacquers poses some problems for the surface
appearance of the product. Indeed, a protective lacquer
film degrades slowly over time and eventually alters the
surface appearance, usually negatively. This film becomes
yellow and can ultimately disappear after several months.
Some films can be hardened with a thermal treatment and
provide longer life. However, complete protection cannot
be achieved with lacquers.
*
Corresponding Author:
Dr. Pierre-Antoine Gay
Ecole d’Ingenieurs du Canton de Neuchâtel-EICN
Dept. de Microtechnique
7, Av. de l’Hotel-de-Ville
CH-2400 Le Locle Switzerland
E-mail: [email protected]
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Oils
Various oils or waxes can be dissolved in perchlorethylene, familiar as a common dry cleaning solvent. Objects
to be covered are dipped into the solution and successively
removed. After the perchlorethylene evaporates, the protective coating remains on the surface and protects the
object from tarnishing. By a simple water or soap rinse,
these oil films can be completely eliminated. This method
is suitable for protecting details in storage. However,
because of the low thickness of the film, any handling of
the treated objects should be avoided.
Inorganic Deposits
At the moment, inorganic or metal coatings deposited by
CVD/PVD, electrochemically or by immersion are most
frequently used to prevent silver tarnishing. These methods can be classed into the following primary areas.
Chromate Conversion Coatings
Chromate conversion coating processes involve immersion of an object in a solution containing chromic acid,
chromates and/or dichromates as primary ingredients. For
example, the film obtained from a potassium dichromate
bath insures ideal surface protection against tarnishing.
Much work has been done in this area. Lacourcelle, et al.1
described a bath containing:
• Potassium dichromate (K2Cr2O7)
• Potassium hydroxide (KOH)
• Potassium carbonate (K2CO3)
20 g/L (2.67 oz/gal)
20 g/L (2.67 oz/gal)
40 g/L (5.34 oz/gal)
The solution concentration, temperature, pH and treatment
time should be taken into account to avoid the appearance
of an undesirable colored film that is detrimental to the
final appearance.
Randin, et al.8 studied the electrochemical chromate
conversion process as an anti-tarnish treatment on a variety
Nuts & Bolts:
What This Paper Means to You
Work in nanotechnology is opening up new avenues in our
industry, but meanwhile, silver still tarnishes. This age-old problem affects areas ranging from electronics to jewelry. Here, the
authors have surveyed the work that has been done and is ongoing, to give us a tour de force of the means of preventing, if not
eliminating the tarnishing of silver.
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of substrates (In, Au, Ag and Cu) used in ornamental applications.
The influence of the composition of the bath as well as that of other
operating parameters was studied. The tarnish resistance of the
chromate conversion coatings was investigated, in particular, after
a weak abrasive treatment. The chromate conversion coating did
not alter the surface appearance of any of the substrate materials
studied and provided very effective protection against tarnishing.
It could be cleaned in the isopropanol with no loss in tarnish resistance. However, it did not withstand the abrasive effect of polishing to the degree generally employed by jewelers. Chromating
remains one of the most effective solutions for the protection of
silver workpieces against tarnishing. However, the use of hexavalent chromium chemistry in the jewelry and silversmith trades is
strongly prohibited because of its high toxicity.
Oxides
Many studies reported in the literature deal with the possibility of
depositing a highly insulating film on silver objects (e.g., oxides
of the III-V elements). Price and Thomas9 proposed an effective
way to avoid tarnishing. This process involved an anodization in
an acidic solution (pH = 5.83) at approximately 5.0 A/dm2 (46.5 A/
ft2). The results of tarnishing tests were better than those obtained
with chromate conversion coatings. A German patent10 describes
oxide deposition by sputtering. Substrates were preheated between
100 and 200°C (212 and 392°F). Hard coatings of silica or alumina
were deposited by sputtering at a pressure between 0.1 and 10 Pa
(1.45×10-5 and 1.45×10-3 lb/in2). The layer thickness was between
0.1 and 10 µm (4.0 to 394 µ-in) [preferentially between 0.8 and 2
µm (31.5 and 78.7 µ-in)].
Tin (Sn)
Among other chemical processes it is important to note the tin
chloride solution treatment.1,11 Tin is used to protect silver objects
because its color and hue are very similar to those of silver. Three
methods of tin coating are usually cited.
Tin tempering. Objects to be protected are immersed in a bath
containing 9 to 23% metal fluoborate solution (the metal being less
noble than silver, such as Cr, Pb, Ni, Co, Cd, In, Zn or Sn). After a
visible film appears, usually in about 5 sec, the parts are removed
from solution. This method is described in a U.S. patent.12
Degreasing – Tinning. Pieces to be coated are immersed in an
alkaline hydroxide bath containing 0.5 to 10% of a bivalent organic
or inorganic tin compound. The substrates become covered with
a very adherent layer which protects against attack by hydrogen
sulfide. This process is described in a German patent.12
Electrolytic tin deposition. Electrolytic tin coatings are used
on a variety of silver alloys containing Cd, Zn, Al and Ag.13 This
method is used mainly for non-decorative applications because the
surface appearance after tinning differs from that of pure silver.
Electrolytic Silver Alloy Deposits & Others
A great number of silver-based coatings are cited in the literature.
They are primarily intended for improved chemical resistance to
hydrogen sulfide. However, most of these alloys are more sensitive
to tarnishing than pure silver. Improved chemical resistance to H2S
is observed for some silver alloys such as Ag-Fe, Ag-Sn and silver
alloyed with the platinum group metals.
Davitz, et al.14 proposed silver alloys consisting of In (17 to
22%), Pd (24 to 27%), Cu (5 to 30%) and Ru (0.25 to 1%). The
ruthenium alloy can be heated to 1177°C (2150°F). It is used in
jewelry and dental applications.
Sasaki and Nishiya15 developed silver alloys containing various
elements including Ag, In (0.2 to 9 wt%), Al (0.02 to 2 wt%) Cu
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(0.3 to 3 wt%) and/or Cd, Sn, Ga and/or Zn (0.01 to 6.5 wt%).
These alloys can be used for decorative applications, but they
need to be annealed between 220 and 900°C (428 and 1652°F)
for 30 min. Finally, silver alloys containing about 6 wt% In, 1.5
wt% Al and 3 wt% Cu show improved results in tarnishing tests
in aqueous solution containing 5% NaCl or 0.1% Na2S. However,
a silver alloy of similar composition (7 wt% In, 1.5 wt% Al and 4
wt% Cu) degrades rather quickly after immersion in 0.1% Na2S
aqueous solution for 10 hr. This example illustrates the complexity
of silver tarnishing chemistry, and in particular the importance of
the chemical composition of the silver-based alloy which can best
retard tarnishing.
Electrolytic composite coatings have been studied with an eye
toward tarnish resistance. For this purpose, De Bonte, et al.16 studied the incorporation of fine germanium oxide (GeO2) particles in
electrolytic deposition processes. Silver alloy coatings containing
up to 6 wt% germanium were electrodeposited from a cyanide
solution without brighteners. Codeposition of germanium with
silver improves tarnish resistance in the thioacetamide test (ISO
4538) when compared to pure silver. The coating surface became
yellowish instead of darkening, and the corrosion products could
be easily removed by simple water rinsing.
Similar results were obtained for silver-germanium and silvercopper-germanium alloys studied by Rateau.17 In two cases, it
appears that the formation of Ag2S layers was inhibited by a protective effect caused by GeOx species.
Incorporation of fine PTFE particles by electrolysis was studied
to improve the tarnish resistance of silver and its alloys by Gay2
and Puippe.18 In some cases, silver coatings with PTFE contents up
to 25 vol% were found to be good candidates to block the formation of Ag2S.
The use of rhodium, palladium and platinum coatings for silver
tarnish protection is strongly suggested for luxury jewelry items.12
The use of zinc and the cadmium, on the other hand, does not show
promising results.12 As for mercury, its use is prohibited because of
its high toxicity.
Organic Inhibitors
Lohmeyer, et al.19 proposed inhibitors based on long-chain aliphatic
thiocyanates as giving suitable protection. Inhibitors dissolved in
an organic solvent are deposited on the surface of the object. The
deposited layers had no significant effect on the electrical contact
resistance whereas their tarnish resistance and tribological behavior were improved.
The use of the cyanuric acid as a silver tarnish inhibitor has been
suggested.20 This inhibitor together with other detergents containing peroxides or whiteners can be used in dish-washing formulations.
Recent work by Moeller, et al.21 quoted the use of acids or
some hydroxamic salts as tarnish inhibitors. For example, 2 vol%
Me(CH2)4CONHOH and CONHOH used in detergent formulations,
provides complete protection of silverware against tarnishing.
According to Franey, et al.22 the use of polymeric materials, socalled reagent polymers, presents interesting results. The polymers
consist of a mixture of resins (polyethylene, polypropylene, ABS
and PVC) and stable additives in solid state in form of bags, tubes,
patches or sheets. The authors claim high protection efficiency in
aggressive atmospheres such as hydrogen sulfide (H2S), carbonyl
sulfide (COS) and hydrogen chloride gas (HCl).
Conclusion
The best protection against silver tarnishing is with the use of
chromate conversion coatings.1 However, this method cannot be
employed in the silversmith trade because of chromium toxicity
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4/30/04, 8:52:09 AM
considerations. Using a silver-based alloy with improved chemical
stability is a good alternative. An efficient though expensive solution can be obtained by rhodium or platinum plating.
Electrodeposition of silver coatings incorporating organic or
inorganic particles has only been mentioned in a few studies to
date. In light of this, it would be interesting to study the tarnish
resistance of certain novel composite coatings, including Pd or
PTFE particles incorporated into a silver matrix. It should also be
noted that, because of the low cost of this material, its use for silver
protection will present a significant commercial benefit.
Acknowledgement
The authors acknowledge the collaboration of CETEHOR** for
financial support and technical help in this research.
References
1.
L. Lacourcelle, Traité de galvanoplastie, Galvano-Conseils
édition, 1996; p. 408.
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résistance à l’usure & à la sulfuration, Thesis, University of
Besançon, Besançon, 2002.
3. K. Hallett, D. Thickett, D.S. McPhail & R.J. Chater, Applied
Surface Science, 203-204, 789 (2003).
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(http://www2.umist.ac.uk/corrosion/JCSE/Volume1/paper14/
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recherche de Suisse, Microélectronique et Horlogerie de
Neuchâtel, Switzerland, 1992.
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edition, 63, 29 (1938).
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Wuerttembergische Metallwarenfabrik AG, European patent,
Allemagne (1996).
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Alloys, AESF, Orlando, FL, 1986.
13. H.H. Uhlig, Corrosion Handbook, J. Wiley & Sons, Hoboken,
NJ, 2000.
14. D. Davitz, U.S. patent 4,895,701 (1990).
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patent (1990).
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Finishing, 80, 43 (July 1993).
17. M. Rateau, Matériaux & Techniques, 10-11, 53 (1995).
18. J.C. Puippe, Swiss patent, 1988.
19. S. Lohmeyer, Galvanotechnik, 84, 762 (1993).
20. P.A. Angevaare & R.G. Gary, U.S. patent 5,468,410 (1995).
21. H. Moeller, H. Blum & C. Nitsch, European patent, Allemagne
(1997).
22. J.P. Franey & K. Donaldson, Proc. MRS Symposium, Materials
Research Society, Warrendale, PA, 1992.
Pierre-Antoine Gay
Patrice Bercot
Jacques Pagetti
About the Authors
Pierre-Antoine Gay is affiliated with the Ecole d’Ingenieurs du
Canton de Neuchatel-EICN, Dept. de Microtechnique in Le Locle,
Switzerland. He holds a DEA degree in material and surface science from the University of Franche-Comté.
Patrice Bercot is engineer and the supervisor of the research
team on pulsed currents and composite coatings at the Laboratory
of Chemical Material and Interfaces, Corrosion and Surface
Treatments of the University of Franche-Comté. He is also
professor at the Superior National School of Mechanics and
Microtechniques, 26 Chemin de l’Epitaphe 25030 Besançon Cedex
France.
Jacques Pagetti is a professor at the Laboratory of Chemical
Material and Interfaces, Corrosion and Surface Treatments of the
University of Franche-Comté. He is a specialist in electrochemistry,
corrosion, metallurgy and surface treatments.
**
Centre Technique de l’Industrie Horlogère, 39 Avenue de l’Observatoire
25000 Besançon, France.
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